An E.coli cell uses run-and-tumble motion to climb up the concentration gradient of a nutrient or a chemical attractant [1]}, [2]}, [3]}. This directed migration, guided by chemical environment surrounding the cell, is known as chemotaxis [4]}. The signaling network inside an E.coli cell consists of two principal modules: sensing and adaptation [5]}. These two modules are coupled to each other via the activity of the chemoreceptors. When the transmembrane chemoreceptors bind to the attractant molecules, their activity decreases. This input signal is processed by the sensing module of the network, and the run-and-tumble motion of the cell is in turn modulated such that the cell shows a net drift towards the region with higher attractant concentration [6]}, [7]}, [8]}. The adaptation part of the network implements a negative feedback mechanism to ensure that the receptor activity does not get too high or too low. This is done by controlling the methylation level of the chemoreceptor such that the activity level is restored to its adapted value. An important aspect of the chemotactic signaling network is the cooperativity or clustering tendency of the chemoreceptors, which allows them to form clusters or `signaling teams' [9]}. All receptors in a team change their activity in unison which results in significant amplification of the input signal. This makes it possible for an E.coli to sense even weak concentration gradient of the attractant [10]}, [11]}.
